Olefins atomic charges

TABLE 6.3. Atomic Charges (me) and NMR Shifts" of Olefinic Carbon Atoms... 

Computed atomic charges and binding energies from an INDO molecular orbital study of a-heteroatom nitrenes are consistent with their known reactivity towards olefins to give aziridines. Molecular orbital calculations on the reaction of propylene oxide and isobutylene oxide with hydrogen chloride and ammonia predict an orientation of addition in agreement with experiment. 

Surprisingly carbonyl-substituted carbanions of phosphonates, in which the negative charge is delocalized over two oxygen atoms, are much more nucleophilic than the corresponding phosphoranes. This effea has first been observed by Homer, and has often been utilized in the synthesis of acylated olefins (R.D. Clark, 1975). 

The difference in concerted reactions is large (5.5 kcal/mole) and no additional factor is needed to explain the high selectivity of the reaction. However, charge-transfer from olefin to aromatic should theoretically be an important stabilizing configuration in this reaction, and one should see if inclusion of CT will modify the conclusion. The charges on the atoms are shown in 17 after transfer of one electron. A contribution of a CT configuration would help to stabilize the preferred orientation relative... 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

The base thus formed from the olefin has a negative charge on a carbon atom, and so may be referred to as a carbanion. The allylic hydrogen is the most acidic because removing it results in a resonance-stabihzed intermediate. 

Selected results are presented in Table 6.3 for typical olefinic carbon atoms. The Mulliken net charges were obtained [40] from full (geometry and exponents) optimizations in the STO-3G basis. 

Figure 6.4. Comparison of NMR shifts, ppm from TMS, with the corrected carbon total (o- + tt) net charges [Eq. (5.10)] reported in Table 6.3 for selected olefins. The atom numbering is that indicated in Table 6.3. The radius of the circles represents an uncertainty of 0.7 ppm or...

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